Cathode-ray apparatus and method



March 28, 1961 K. J. GERMESHAUSEN ETAL 2,977,501

CATHODE-RAY APPARATUS AND METHOD 2 Sheets-Sheet 1 Filed June 14. 1956 5 mmew m T R A N E R w 0 O E V G L D W miwww HRH m? OE mam KSD Y H B H V H 00 F l o 1 0 m. m m h h u m T n. r

March 1951 K. J. GERMESHAUSEN El'AL 2,977,501

CATHODE-RAY APPARATUS AND METHOD 2 Sheets-Sheet 2 Filed June 14. 1956 m m m m KENNETH J. GERMBHAUSEN SEYMOUR GOLDBERG DANIEL F. MC DONALD A TI'OR/VEKS CATHODE-RAY APPARATUS AND METHOD Kenneth J. Germeshausen, Newton Center, and Seymour Goldberg, Lexington, Mass, and Daniel F. McDonald, New York, N.Y., assignors to Edgerton, Germeshausen & Grier, Inc., Boston, Mass, a corporation of Massachusetts Filed June 14, 1956, Ser. No. 591,339

7 V 17 Claims. c1. 315-17 The present invention relates to cathode-ray apparatus and methods of operating same, and, more particularly to cathode-ray apparatus of high sensibility for indicating transients in the milli-microsecond region.

The oscillographic recording of millimicrosecond transients presents difficult problems not only because of the wide system-bandwidth required but, also, because the writing or recording speeds are very high. As an illustration, the bandwidth requirement may extend at least to one thousand megacycles per second in order to accommodate pulse rise times of the order of a tenth of a mini-microsecond. At the same time, a recording speed of the order of trace widths per second is required in order to produce a readable record for interpretation. Cathode-ray tubes of the traveling-wave deflection type have been developed to afford along effective deflectionplate length without introducing transit-time distortion, thereby providing high-beam velocities in the deflection region that assist in meeting the wide bandwidth requirements. Unfortunately, however, such tubes have been limited to deflection sensitivities sufiicient to resolve signals of the order of a half a volt. There are many applications, of course, that require much higher sensitivities and the problems attendant upon such applications can not be solved with the aid of present-day cathode-ray tubes. While the problem could be approached by developing an amplifier with the required bandwidth and voltage output to drive such cathode-ray tubes, such a development involves considerable complexity. Attention has therefore been concentrated upon improving the sensitivity of the cathode-ray tube system itself through improving the actual electron-optical operation therein.

An object of the present invention, accordingly, is to provide a new and improved cathode-ray apparatus that shall embody improved electron-optical operation, providing vastly increased sensitivity.

A further object is to provide a new and improved method of operating cathode-ray apparatus to provide such markedly increased sensitivity, particularly at the high speeds required in the above-mentioned transient recording systems.

Still another object is to provide novel elements suitable for use in such cathode-ray apparatus.

Other and further objects will be explained hereinafter and will be more particularly pointed out in the appended claims.

In summary, it has been discovered that vastly im proved cathode-ray-tube sensibility, as hereinafter defined, can be achieved through the employment of focusing and deflection fields in overlapping space relationship, preferably with substantial coincidence of the central regions of the focusing and deflection fields in the cathode-ray apparatus. The focusing field is adjusted, moreover, to obtain a magnification substantially equal to the ratio of a predetermined minimum resolvable phosphor spot-cross dimension to the cross dimension of the point source of electrons produced in the cathode-ray apparatus. Pre- 2,977,591 Patented Mar. 28, 1961 ferred constructional and operational details are hereinafter more fully discussed.

The invention will now be explained in connection with the accompanying drawings,

Fig. l of which is a schematic diagram of a cathoderay tube electron-optical system designed in accordance with some of the principles of the present invention; and

Fig. 2 is a perspective view of a cathode-ray apparatus embodying the invention in preferred form, with parts being shown broken away to illustrate details of construction.

Referring to Fig. 1, a cathode-ray-tube apparatus is shown comprising an electron gun for producing a point source of electrons comprising a cathode 1 from which electrons are emitted, a control-grid electrode 3 having an aperture 5 through which the electrons produced by the cathode 1 are passed in response to appropriate accelerating voltages, and an anode electrode 7 having an aperture 3 through which the accelerated electrons may pass. The electron gun comprising the cathode 1, the control-grid electrode 3 and the anode 7 is, in effect, an immersiontype electron lens in which the electron beam is formed into substantially a point source 2, known as the crossover point, between the control-grid electrode 3 and the anode 7. The diameter of the point source of electrons at the cross-over point 2 is represented by the symbol d Measurements on existing cathode-ray tubes indicate that cross-over diameters a of the order of 0.001 inch or 1 mil are readily obtainable. The electron beam is shown diverging from the cross-over point 2 through the aperture 9 in the anode 7 in response to anode voltage. None of the conventional operating voltages are shown applied to the various electrodes in this schematic diagram but details concerning the necessary values of voltages will be explained hereinafter. The electron beam continues through an aperture 13 in an aperture stop or limiting device 11 and thence into the fields of a main electronfocusing lens, indicated by the concentric dash-line circles 17, and an electron-beam deflection apparatus, indicated by a pair of parallel spaced deflection plates 15, finally to impinge upon a phosphor particle 23 on a fluorescent phosphor viewing screen 21. The application of an ap propriate deflection voltage to the deflection field-producing means 15 will cause the stream to impinge upon the phosphor particle 23 at any desired point 19 located any predetermined distance d from the center axis 0 of the screen 21. The angle of deflection 0 between the deflected. beam and the axis 0 is proportional to the deflection voltage. The focusing field 17 converges the electrons into a point when the beam strikes the phosphor particle 23 of the screen 21. In the case of a magnetic focusing field 17 of relatively thin dimensions, the magnification produced in the electron-optical system, represented by the symbol M, is substantially equal to the distance Q from the focusing field 17 to the screen 21, divided by the distance P from the cross-over point 2 to the focusing field 17.

In systems employing photographic-recording and similar techniques that permit magnification of the cathoderay-tube display, the pertinent characteristic for measuring the degree of usefulness of the tube is the number of volts required to deflect the electron trace through one trace width. This is the definition of the sensibility before referred to. Another term hereinafter employed is the writing speed, which is defined as the number of trace widths per second. The full scan produced on the cathode-ray-tube fluorescent screen is also measured in terms of trace widths. In the design of optimum sensibility cathode-ray-tube systems, the above factors are all interrelated, and it is necessary to adjust the design parameters to achieve an optimum relationship between sensibility, writing speed and scan.

While the deflection means 15 could be placed anyesteem where in the electron path between the cathode 1 and the screen 21, if it is disposed between the focusing field 17 and the screen 21, the Sensitivity in the operation of the tube will increase as the deflection means is moved back tow ar ds the focusing field 17. If, however, the deflection means isdisposed between the focusing field 17 and the cathoded, movement of the deflecting system toward the focusing field 17 will cause successively increasin shifts in the position of the crossover point 2. The actual deflection d along the 1 screen 21 is determined by' this shift in the cross-over point 2 multiplied by the'magnification M of the electron-optical system. The greatest shift in position of the cross-over point 2 is'achieved when thed'eflection means 15 is farthest'from thecathode 1 or closest to the focusing field 17.

' It has been discovered that, for the purposes of the present invention, the optimum "position of the deflection means 15, contrary to the position presently utilized in cathode-ray apparatus, is squarely within the central region of the cathode-ray electron-lens or focusing field 17. Desirable results can beobtained in any position of overlapping relationship between the deflection and focusing fields, 'but, asbefore indicated, the coincidence of 'thecentra'l regions'of the deflection and focusing fields has been found to produce optimum results. If the focusing field 17 is created by a magnetic electron lens, such as af-ocusing coil,'there is no physical limitation in' positioning th'eldeflection'means 15 in close proximity to thefocusing field and, in fact, with the optimum-condition central-region coincidence. With electrostatic focusi'ngffilds, however, there is a limitation on how close the deflection means 15. may be located to the focusing field in'view of the physical construction of an sis. ir s i ens.

erion underlying, the present invention, accord gly, sides i "the "substantial overlapping and preferable central-'1" 'hf'coiiicidence of the deflection and focusingfields 1'5 and 17, a's'sch'ema'ticaly illustrated in Figfl. As before indicated, thisis the very antithesis "of the arrangement of present-day cathode-ray apparatus in whicjhthe deflection means and focusing means are considerably spaced apart, and it has been found 'to produce thesensibility required to obtain optimum results in high-speed 'oscillog'raphy and recording. "In practice, moreover; ithas befen'fojurid that'the efiective'defle-ction field'in theffocu'sing field region should be restricted to less than about ten'ip'ercentofthfe total transverse dimension of the focusing field 17 in order to avoid entering the aberrant portions of the focusing field.

"With the reflection position thus optimized by its location within the focusing field 17, the problem then is to determine the optimum electron-optical magnification or position of the focusing field or lens 17. When the focusing field 17 is located farthest towards the cathode 1, maximum sensitivity is achieved but'only at the 'expense'of increasing the spot size of the'electron beam upon the phosphor layer '23 of the screen 21. This 'is because the magnification M is maximum at such. a location of thefocusing field. With the focusing field 17 closest to the screen '21, on the other hand, minimum magnification and the smallest 'electi ombeam spot are produced, but Wtih very low sensitivity. Since, for the applications of the cathoderay apparatus of the present invention, the primary concern is the sensibiity factor, that. is, the number ofvolts required to deflect the electron beam one spot diameter upon the screen 21, it is necessary to determine what the effect ofm'ag'nification and other parameters may be upon such sensibility. From theoretical and experimental data, the following relation 'has been evolved'forth'esensibiilty S, of the cathode-ray apparatus of Fig.1:

Where. Y istheelectron beam voltage, Sis the electronbeam scan expressed in number of spot widths at the screen, L is the length of the cathode-ray tube, 1 is the length of the deflection means, L is the minimum value of light energy emitted per unit area of screen phosphor for film-recording purposes, v is the minimum writing speed required, A and n"-'are screen phosphor characteristics, V is the screen voltage, vj is the eurrent per unit solid angle in the beam, and the remaining terms have the definitions before presented.

From the above equation, it is evident that a reduction and hence an improvement in the'sensibility S results directly from a reduction in the magnification"M. The lower usable limit of the magnification M, however, will be determined by the screen resolution. If the minimum resolvable spot diameter is represented by d then the magnification M should be adjusted, in accordance with the present invention, to produce a spot of this diameter; that is, such that The screen resolution, therefore, imposes the primary limit upon the system performance. Experience with fine-grain phosphors, as used on image converters and the like, indicates that a resolution of at least twenty lines per millimeter can be obtained. This corresponds to 'a line width of 0.601 inch or 1 mil. Ordinary P-ll type phosphors will resolve about one-half this number of lines. The minimum recordable linewidth upon the faster photographic films is also of theorder of 1 mil. If, accordingly, the value of 1 mil is utiliied as the minimum line or spot width resolvable "on the phosphor, the focusinglens 17 together with the overlappingdeflection means 15 must be moved sufiicie'ntly close to-"the screen tlir give a magnification M of substantially unity. This isbeca'u'se, as before stated, the Cross-Over diameter "d rhension of the cross-over point 2 of the electron beam. 'InF-ig. 2, physical structures, schematically illustrated in Fig. 1, are shown in preferred form. -In the upper right-hand corner, the electron gun is illustrated as comprising a conventional electron-emitting cathode element 1, a cylindrical control-grid electrode 3 having a central aperture 5, and a first anode 7 having a central aperture 9." Instead of the three-element electron gun 1, 3, 7 of r Fig. l, the electron gun of Fig. 2 includes a'fourth anode in the form of a longer hollow cylinder 4. In accordance with a further feature of the present invention, this second anode 4 is employed to decelerate the electron beam beyond the cross-over point 2 to permit the presence of lower beam velocities in" the deflection region of the defiection means 15. Again, for purposes of simplicity, the actual leads connecting these electrodes to voltage sources are not illustrated. The cathode 1 may be operated by connection, for example, to a source of voltage .of, say, 1400 volts. The control-grid electrode 3 may be operated with a grid-driving voltage of the order of 200 volts, having a cut-off voltage in a range of from 250 volts 'to 600 volts. The first anode 7 may be connected to a source of voltage ranging from 0 to 2600 volts. The second anode, however, which, as bef' re stated, decele'rates the electron beam beyond the crossover point 2, may be grounded so that it is at a lower voltage than the voltage of the anode 7. In accordance with the present invention, again as contrasted with priorart cathode-ray-tubes, ithas been found that the hearse beam velocities may be lowered without altering the position or size of the crossover point 2 if the lower-voltage anode 4 is disposed immediately adjacent to the highvoltage anode 7. In such a case, the lens effect of the second anode 4 upon the crossover point 2 is minimized so that the cross-over point 2 remains substantially in the same position and of the same size as if the lowvoltage second anode 4 were not employed. The advantage has been gained, however, that, without altering the position or size of the cross-over point 2 of the point source of electrons, lower beam velocities are produced in the deflection region, thereby increasing the sensitivity of the deflection system.

The electron-gun assembly is shown supported from the anode cylinder 4 by means of insulated struts 8 within an evacuated glass envelope 10. The cathode 1 may be energized by leads 8', passed through the end seal or press in the envelope 10. The envelope 10 itself may be joined by glassing to a Kovar cup 12 which, in turn, connects with a conducting stainless steel or other tubular member 14 having a diameter of, say, one and a half inches. The Kovar cup 12 may be copper-brazed to the tube 14. Disposed within the tube 14 is the limiting aperture plate or stop 11 having a very narrow substantially rectangular aperture 13 the purpose of which will now be explained. While conventional cathode-ray apparatus utilizes circular apertures for accurate alignment with the signal deflection means, in the present invention, the limiting aperture plate 11 is made part of the deflection system assembly 14 and the aperture 13 is made of greater vertical than horizontal dimension. The

aperture 13 is aligned to reduce the electron-beam thickness in the direction of signal deflection. This technique permits closer spacing of the signal deflection means 15 while permitting higher total beam current than can be obtained with the usual conventional circular aperture of smaller cross-section. A typical successfully operated aperture of this characterhad dimensions 0.030 by 0.100 inch.

Instead of mere parallel deflection plates 15, as shown in Fig. 1, it is deemed preferable to minimize the effects.

of transit-time distortion at low-beam energy by employing a traveling-wave helical member 15, Fig. 2, which propagates the signal along the electron path at the same velocity as the beam. Somewhat similar traveling-wave deflection systems are described, for example, by J. R. Pierce, in Electronics, 1949, vol. 22, page 97. The helical member 15 is designed so that the propagation time of the signal around one turn of the helical winding is made substantially equal to the time of transit of an electron along a distance equal to the winding pitch. The helical member 15, actually comprises a pair of helical windings 15a, wound upon a ceramic form 15b to provide balanced deflection. The dimensions of the windings and the spacing to the wall of the tube 14 are chosen to provide an impedance of approximately 120 ohms for each section thereof. The axial velocity of the signal along the deflection means 15 is equal to the speed of the resulting substantially 1.4 kilovolt electron beam, in the above illustration. The two helices 15a may be substantially three inches long and their separation, which corresponds to the deflection-plate separation, is about 0.120 inch. The signal and electron transit time for that three-inch length of the deflection system is '.i. l l0- seconds. Coaxial deflection-plate lead-in supports 18, are provided for supplying deflection voltage. These lead-in supports may be brazed to the stainless steel or other conducting tubular body 14 and connection may be made to the deflection members by a Kovar wire 20. This type of assembly enables convenient attachment of highfrequency coaxial lead-in connections and also provides a high degree of ruggedness and dimensional accuracy while maintaining a relatively small diameter of tube 14 "that greatly eases the magnetic focusing lens requirewant.

The magnetic focusing lens is shown in the form of a pair of focusing-coil rings 17 which, in accordance with the present invention, are centrally mounted along the deflection apparatus 15. The deflection rings 17 may be conventional magnetic coil deflectors disposed in series. The object and image distance are then nearly equal to the focal length of each lens. Each lens, moreover, is then working at its designed optimum magnification. Just beyond the signal-deflection system 15 is disposed a pair of timing sweep or scan deflection plates 30. Since high sensitivity is not required for sweep deflection, the electron input and output ends of the sweep plates 30 are rounded, as shown at 32, to eliminate astigmatic effects that occur because of the proximity of the electron beam to the plates. The sweep plate separation may be 0.150 inch and the length may be 0.6 inch.

Sweep voltages may be applied by' the coaxial lead-in supports 16.

A further feature of the present invention resides in the employment of post-deflection acceleration provided by a high-resistance spiral 22, as of aquadag, deposited inside the insulating tubular screen support 24. The tubular support 24 may be sealed by Kovar-ring seals 34 and 26 to the left-hand end of the tube 14 and to the flat-disc phosphor-screen 21, respectively. A co-axial high-voltage Kovar lead-in member 28 may be provided at the ring 26, the aquadag spiral 22 being connected at its ends between high voltage and ground. A final anode cylinder 36 may surround the member 24. If the highvoltage post acceleration had been effected in a limited region near the screen 21, a strong lens action would have been exerted upon the electron beam that would have markedly reduced the deflection sensitivity. Through applying the post-deflection acceleration along a long high-resistance path 22 distributed between the screen and the deflection means and of actual electrical length far greater than the physical distance from the screen 21 to the deflection means through the utilization of the spiral 22, such focusing or lens action is greatly reduced and minimum loss in sensitivity results. The construction of the present invention, moreover, has further advantages over conventional post-deflection systems in that a single spiral is utilized instead of the priorart multiplicity or sets of bands for distributing the accelerating potential in the post-deflection region between the deflection means and the screen 21. Such prior-art systems employ, for example, three to five bands for distributing the post-accelerating potential, requiring a multiplicity of lead-in connections and an external voltage divider and still being appreciably subject to the beforementloned undesirable lens action, all of which dilficulties are eliminated in accordance with the present invention.

With the tube constructed as above explained, sensibilities of the order of 0.026 volt per trace width have been obtained which are from 10 to times better than the sensibilities obtainable with the prior-art highfrequency recording cathode-ray-tube systems. A signal scan of 300 trace widths has been used with the system of Fig. 2 with a sweep-plate deflection factor of about volts per inch. The sweep scan has been of the order of 500 trace widths with a writing speed of 10 trace widths per second. The trace width itself has been of the order of 0.0012 inch, and the transit time of the order of 2X 10' seconds. The band width of the device may be of the order of 450 megacycles per second. Through redesign of the helix structure of the deflection mechanism 15, however, the band width may be raised to the 1000 or 2000 megacycle value frequently required, as before explained.

While the invention has been described in connection with a screen 21 having a phosphor layer 23, it is to be understood that the teachings herein disclosed may also be applied to other types of cathode-ray apparatus including those having other types of screens or targets.

Modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention, as defined in the appended claims.

What is claimed is: l. Cathode-ray apparatus having, in combination, an electron gun for producing a point source of electrons, a

screen, electron-focusing means disposed to extend only in a region intermediate the source and the screen for producing a field for focusing the point source of electrons upon the screen, and deflection means disposed in close field-overlapping proximity to the focusing means for producing a field for deflecting the focused electrons along the screen, the deflection means being disposed electron gun for producing a point source of electrons, a

fluorescent phosphor screen having a predetermined minimum resolvable phosphor-spot cross-dimension, electronfocusing means disposed so that the ratio of its separation from the screen to its separation from the point source of electrons is sufliciently small to produce a field for focusing the point source of electrons up the screen as a spot of cross-dimension corresponding substantially to the said minimum resolvable phosphor-spot cross-dimension, and deflection means disposed in overlapping relation with the focusing means for producing a field for deflecting the focused electrons along the screen, the deflection means being disposed well within and displaced transversely from the focusing means in order to limit the effective transverse dimension of the deflection field to afraction of the total transverse dimension of the focusing feld, thereby to avoid having the deflection field enter the peripheral aberrant portions of the focusing field.

3. Cathode-ray apparatus having, in combination, an

electron gun for producing a point source of electrons of predetermined cross dimension, a fluorescent phosphor screen having a predetermined minimum resolvable phosphor-spot cross-dimension, electron-focusing means disposed sufliciently close to the screen between the screen and the point source of electrons to provide a field for electron-optical magnification substantially equal to the ratio of the predetermined minimum resolvable phosphorspot cross-dimension to the point-source predetermined cross dimension, and deflection means disposed with its central region substantially coincident with the central region of the focusing means for providing a field for deflecting the focused electrons along the screen, the deflection means being disposed well within and displaced transversely from the focusing means in order to limit the eflective transverse dimension of the deflection field to a fraction of the total transverse dimension of the focusing field, thereby to avoid having the deflection field enter the peripheral aberrant portions of the focusing field.

4. Cathode-ray apparatus as claimed in claim 1 and in which the dimension of the deflection field in a plane transverse to the direction of travel of the electrons is less than substantially ten percent of the corresponding transverse dimension of the focusing means in the region thereof.

5. Cathode-ray apparatus as claimed in claim 2 and in which the dimension of the deflection field in a plane transverse to the direction of travel of the electrons is less than substantially ten percent of the corresponding transverse dimension of the focusing means in the region thereof.

6. Cathode-ray apparatus as claimedin claim 2 and in which a limiting aperture is disposed between the point source of electrons and the deflection means, with the length of the aperture along the direction of deflection of the electron beam along the screen being greater than the width of the aperture.

7. Cathode-ray apparatus as claimedin claim '6 and in which the said limiting aperture isof substantially rectangular configuration.

8. Cathode-ray apparatus as claimed in claim- 2 and in which the electron gun comprises an electron-emitting cathode, acont-rol electrode and a high-voltage apertured anode'for converging the electrons from the cathode to form the said point source in the region or the aperture of the high-voltageanode, and in which a lower-voltage electrode through which the electrons may pass is disposed immediately adjacent the high-voltage anode to reduce the electron velocity without substantially altering the position or size of the said point source of electrons.

9. Cathode-ray apparatus as claimed in claim 2 and in which the deflection means comprises helical conductor windings.

10. Cathode-ray apparatus as claimed in claim 9 and in which the helical conductor windings are in the form of'two traveling-wave helices wound upon ceramic forms to provide balanced deflection.

11. Cathode-ray apparatus as claimed in claim 2 and in which a high accelerating voltage is applied in the region of the screen, a low-voltage anode is disposed close to the deflection means, and a high resistance path isprovided between the screen and the low-voltage anode the length of the path being greater than the distance from the screen to the deflection means in order to reduce focusingaction therebe'tween.

7 l2. Cathode-ray apparatus as claimed in claim 11 and in which the. high-resistance pathcomprises a spiral between the screen and the/said, low-voltage anode.

1'3. Ametho'd'of operating cathode-ray apparatus provided with a phosphor screen having a predetermined minimum resolvable phosphor-spot cross-dimension, that comprises, producing a point source of flowing electrons of predetermined minimal cross dimension, producing a focusing field disposed to extend only in a region intermediate the source and the screen to focus the point source of electrons upon the screen, adjusting the position of the focusing field sufficiently close to the screen to provide electron-optical magnification substantially equal to the ratio of'the predetermined minimum resolvable phosphorspot cross-dimensionv to the point source predetermined cross-dimension, applying a deflection field in overlapping space relation with the focusing field with substantial coincidence of the central regions of the focusing and deflection fields, and limiting the effective extent, transverse to the electron flow, of the deflection field within the focusing field to a fraction of the corresponding extent of the focusing field.

14. A method of. operating cathode-ray apparatus pro vided with a screen, that comprises, producing a point source of flowing electrons, producing a focusing field disposed to extend only in a region intermediate the source and the screen to focus the point source of electrons upon a predetermined point, applying a deflection field in overlapping space relation with the focusing field to deflect the electrons from the predetermined point to an adjacent predetermined point, and limiting the size of the deflection field transverse to the electron flow to or under substantially ten percent of the size of the focusing field in the region thereof.

15. A method of operating cathode-ray apparatus provided' with a phosphor screen having a predetermined minimum resolvable phosphor-spot cross-dimension, that comprises, producing a point source of flowing electrons of predetermined minimal cross dimension, producing a focusing field disposed to extend only in a region intermediate the source and the screen to focus the point source of electrons upon the screen, adjusting the position of the focusing field sufiici'ently close to the screen to pro vide electron-optical magnification substantially equal to the ratio of the predetermined minimum resolvable phosphor-spot cross-dimension to the point source predetermined cross-dimension, applying a deflection field in overlapping space relation with the focusing field, and limiting the size of the deflection field transverse to the electron flow to or under substantially ten percent of the size of the focusing field.

16. Cathode-ray apparatus having, in combination, an electron gun for producing a point source of electrons, a fluorescent phosphor screen having a predetermined minimum resolvable phosphor-spot cross-dimension adjusted to correspond substantially to the minimum recordable line width upon photographic films, and electron-focusing means disposed so that the ratio of its separation from the screen to its separation from the point source of electrons is sufficiently small to focus the point source of electrons upon the screen as a spot of cross-dimension corresponding substantially to the said minimum resolv able phosphor-spot cross-dimension.

17. Cathode-ray apparatus as claimed in claim 2 and in which the said resolvable phosphor-spot cross-dimension corresponds substantially to the minimum recordable line Width upon photographic films.

References Cited in the file of this patent UNITED STATES PATENTS Ackermaml Oct. 28, Haeif Dec. 15, Michelssen Apr. 13, Litton June 7, Schwartz July 12, Gray Aug. 26, De Vore Dec. 16, Buckbee Ian. 28, Schade Mar. 8, Pierce Dec. 26, Smyth I an. 30, Kreiger Oct. 23, Law u July 1, Sziklai Feb. 2, Field Nov. 29,

FOREIGN PATENTS Australia Feb. 14, 

